1. Field of the Invention
The present invention relates to a glass material for molding an optical element, a molding product of an optical glass element and a manufacturing method thereof. Concretely, the present invention can be applied to optical parts such as a lens, a prism and a mirror, and a hybrid lens (made of plastic and glass), an optical element of X-ray glass, etc. used in a camera, a plain paper copier (PPC), a facsimile telegraph (FAX), an optical disk drive unit, etc. More particularly, the present invention relates to a glass material for molding an optical element, a molding product of an optical glass element and a manufacturing method thereof in which no lead, etc. are easily deposited from a glass element in hot press molding thereof using a die so that no lead, etc. are easily attached to the die and it is possible to restrain an accuracy in face of the die from being reduced. The molding product of an optical glass element in the present invention can be preferably used for a camera, a copying machine, a facsimile telegraph, a printer and an optical system using an optical disk, a compact disk (CD), a CD-ROM, a lens, a prism, etc.
2. Description of the Related Art
Optical elements using general glass are utilized in various kinds of fields. These optical glass elements are ground to form spherical and aspherical surfaces.
Recently, a press molding method for an optical glass element has been searched and practically used. In this press molding method, similar to plastic, etc., a glass material is heated at a temperature equal to or higher than 400 degrees. Thereafter, the glass material is pressurized to obtain an optical element having a desirable shape. In this press molding method, glass comes in contact with a die processed to have a high accuracy in shape so as to obtain the desirable optical element at a high temperature. A shape of the pressurized die is transferred to this glass. Therefore, heat resistance, acid oxidation resistance and high strength are required with respect to the die.
However, it is impossible to completely prevent the die from being deteriorated even when the die is formed by using various kinds of materials. When an optical glass element is formed by hot press molding a flint glass material having a high refractive index and a high degree of dispersion by using a molding die, ions of lead and potassium as constructional components of glass are deposited from an internal glass portion at hot press molding times. These ions are diffused onto an interface of this glass and the die and react with a small amount of reducing substances such as hydrocarbon existing on a die surface so that metallic lead and metallic potassium are formed. Thus, glass and the die are melted and attached to each other by lead and potassium deposited on the glass surface. Further, a coloring film of lead and potassium is formed on the glass surface so that this glass surface becomes opaque. When the glass surface becomes opaque, transmittance of light is reduced so that it is impossible to obtain an optical glass element having desirable optical characteristics in some cases. Further, when a coating film such as an antireflection film is formed on the glass surface in a state in which the coloring film of lead and potassium is formed on the glass surface, the coating film tends to be separated from the glass surface in comparison with a case in which the coating film is formed on a clean glass surface.
For example, Japanese Patent Publication (KOKOKU) No. 4-21606 (Japanese Patent Application Laying Open (KOKAI) No. 62-207727) shows a technique for solving the above problems. In this publication, a glass element before hot press molding is coated with an aqueous solution of hydrofluoric acid. Thereafter, the coated glass element is cleaned by water and is dried. These cleaning and drying processings are repeatedly performed. Thereafter, the glass element is processed by hot press molding. Further, for example, in Japanese Patent Publication (KOKOKU) No. 4-21606, a glass material before hot press molding is dipped into hydrofluoric acid and is then dipped into nitric acid. This glass material is cleaned by pure water and is dried. Thereafter, the glass material is processed by hot press molding. In both the above cases, a glass surface portion can be etched and a constructional component can be eluted from an internal glass portion by surface processing of the glass material using a solvent before the hot press molding. Accordingly, no glass surface easily becomes opaque and no glass is easily melted and attached onto a die after the hot press molding. However, in these cases, the glass surface is processed by only chemical processing using a solvent without any physical processing such as ultrasonic processing in the present invention as described later. Therefore, effects of this surface processing are not necessarily sufficient and a problem exists in that it is impossible to sufficiently satisfy recent severe requirements about optical characteristics by using the above method, consequently an yield of the optical glass element is reduced.